1. Field of the Invention
The present invention relates to a mobile communication system, a mobile communication method and a mobile communication program in which a mobile station such as a cellular phone communicates with a base station by way of at least one other mobile station.
2. Related Background Art
Various communication schemes have conventionally been employed in digital mobile communication systems. In a system adopting the TDMA (Time Division Multiple Access) scheme, for example, the whole bandwidth is used, a predetermined time period (frame) is divided into a plurality of unit times (time slots), and each time slot is used as a communication channel. The CDMA/TDD (Code Division Multiple Access/Time Division Duplex) scheme in which each time slot is subjected to code division multiplexing has also been proposed.
Modulation modes used in mobile communication systems include those of phase shift modulation such as BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) and 8PSK, and those of quadrature amplitude modulation such as 16QAM (Quadrature Amplitude Modulation) and 64QAM. By way of example, FIG. 4 shows signal points in the QPSK, whereas FIG. 5 shows those in the 16QAM.
The QPSK is a modulation mode for converting digital data to analog signals. It is used for efficiently transferring data in a limited bandwidth, since it expresses information by using both the amplitude and phase of a wave. As shown in FIG. 4, using two amplitude states and two phase states enables 2×2=4 values be represented in a single symbol.
As shown in FIG. 5, the 16QAM is one kinds of multiple value quadrature amplitude modulation capable of transmitting 4-bit (16-value) information with a single symbol. It can transmit a greater amount of information per symbol than the other digital modulation modes do, thereby being able to realize high-speed digital communications with a narrow bandwidth.
In conventional mobile communication systems, one of modulation modes may be set fixedly for each system or a plurality of modulation modes may be used adaptively. In the adaptive modulation mode in general, the amount of information that can be sent at once is increased by using a modulation mode having a greater number of modulation values if the communication quality is favorable, whereas a modulation mode having a smaller number of modulation values is used in order to reduce the influence of noise if the communication quality is unfavorable.
Also known as a transmission scheme is ODMA (Opportunity Driven Multiple Access). The ODMA is a communication scheme in which, as shown in FIG. 6, communications between a base station 50 and a mobile station 51, for example, are carried out by way of other mobile stations 52, 53. In this case, the mobile station 51 located in a low bit rate region can attain a higher bit rate communication by communicating with the base station 50 by way of the other mobile stations 52, 53.